1. Field of the Invention
This invention relates to a control system for a compression ignition internal combustion engine that causes combustion of an air-fuel mixture by self-ignition.
2. Description of the Related Art
Conventionally, a control system of the above-mentioned kind has been proposed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2001-289092. In the engine, the timing for opening and closing an intake valve and an exhaust valve of each cylinder is configured to be variable. Further, in the control system, paying attention to the relationship between the timing of occurrence of self-ignition and the temperature of working medium (working gases) at the start of a compression stroke that the self-ignition timing is advanced as the temperature of working medium at the start of the compression stroke is higher, the temperature of working medium is controlled for control of the timing of occurrence of self-ignition. More specifically, by setting the valve-closing timing of the exhaust valves to be advanced, and the valve-opening timing of the intake valves to be delayed, part of combustion gases is caused to remain in a combustion chamber (internal EGR). Further, the amount of the combustion gases remaining in the combustion chamber                (hereinafter referred to as “the internal EGR amount”) is controlled according to the temperature of the exhaust gases, which is detected by a sensor provided in an exhaust pipe, whereby the temperature of the working medium is controlled. This causes self-ignition to take place in suitable timing, whereby knocking and misfire are prevented from occurring.        
As described above, the conventional control system is configured such that the heat of the combustion gases is utilized to cause self-ignition in suitable timing, and the temperature of working medium is controlled by controlling the internal EGR amount. The temperature of the exhaust gases is used as a parameter indicative of the temperature of the combustion gases. In the control system, however, the sensor for detecting the temperature of exhaust gases is provided in the exhaust pipe, which means that the temperature of exhaust gases already discharged from the combustion chamber is detected by the sensor. Therefore, the temperature of exhaust gases detected by the sensor does not appropriately reflect the temperature of the combustion gases which are to be generated by the following combustion and remain in the combustion chamber. The above difference between the detected temperature of the exhaust gases and the temperature of the residual combustion gases tends to be larger particularly during a transient operation of the engine, since the degree of change in the temperature of combustion gases increases due to changes in operating conditions of the engine.
As described above, when the detected temperature of the exhaust gases is different from the temperature of the residual combustion gases, it is impossible to accurately control the temperature of working medium at the start of the compression stroke even if the internal EGR is controlled according to the detected temperature of the exhaust gases. As a result, self-ignition cannot be caused in suitable timing, which makes it impossible to prevent knocking and misfire from occurring.